As illustrated in FIG. 3, with a conventional optical head 21 used for recording or reproducing information on/from a recording medium, a laser beam emitted by a semiconductor laser 22 is converted into a parallel beam of light by a collimating lens 23 and is split into a transmitted beam and a reflected beam by a beam splitter 24. The transmitted beam is converged upon a recording medium 26 by means of an objective lens 25 and is employed for recording or reading information. As to the reflected beam, it impinges upon a photodiode 29 that is installed between and connected to output terminals 27 and 28, and here is converted into an electric signal. This electric signal is used as detection output for detecting the optical power of the semiconductor laser 22 to be sent by the optical head 21 to an external circuit.
It is a characteristic feature of the semiconductor laser 22 of the optical head 21 that the optical power thereof varies greatly due to changes in temperature, elapse of time, or according to the current applied thereto. In order to obtain a stable optical power, it is therefore indispensable to have a portion of the laser beam emitted by the semiconductor laser 22 received onto the photodiode 29 in a fixed proportion, and to adjust the optical power of the semiconductor laser 22 in accordance with the detection output released by the photodiode 29. Hence, in an optical information recording/reproducing device, an APC (Auto Power Control) circuit, not shown, is usually accommodated outside of the optical head 21 and, using the detection output of the photodiode 29, the optical power of the semiconductor laser 22 is adjusted by means of the APC circuit so that a laser beam having a suitable light intensity is converged upon the recording medium 26.
However, the following difficulty arises when manufacturing the conventional optical head 21. That, is, the detection output that the photodiode 29 releases is different in every optical head 21 manufactured making it difficult to accurately adjust the optical power of the semiconductor laser 22 by means of the APC circuit. The reason why the detection output released by the photodiode 29 varies will be discussed hereinbelow.
The optical head 21 used for recording requires a strong light to perform the recording of information. Consequently, in order to obtain a laser beam having a high power of approximately 5 mW to 15 mW, provision is usually made such that the transmittance of the beam splitter 24 is high and the laser beam emitted by the semiconductor laser 22 is efficiently illuminated upon the recording medium 26. This is due to the fact that, under present mass production conditions, the maximum output of the semiconductor laser 22 is at most approximately equal to 40 mW. It is therefore not possible to obtain a laser beam of a high power unless the beam splitter 24 has a high transmittance.
Here, supposing that the light intensity of the portion of the laser beam that was transmitted through the beam splitter 24 is represented by I.sub.t, the light intensity I of the portion of the laser beam that was reflected by the beam splitter 24 and impinges upon the photodiode 29 is determined by the transmittance T and the reflectance R of the beam splitter 24 according to the following equation: EQU I=I.sub.t .times.R/T
With the present mass production technology, it is difficult to contain the accuracy of the transmittance T and the reflectance R below .+-.5%. For instance, the transmittance T of the beam splitter 24 accommodated in the optical head 21 used for recording is generally equal to 0.7.+-.0.05, and its reflectance R is generally equal to 0.3.+-.0.05. Consequently, according to the above equation, the light intensity I is equal to: EQU I=(0.43.+-.0.1).times.I.sub.t
or, if put in terms of a fluctuation range, can extend to as much as .+-.23%. In other words, supposing that the light intensity I.sub.t is constant, a discrepancy of 1.6 times occurs between the greatest and the smallest detection signals released by the photodiode 29 of the optical head 21. The light intensity I.sub.t stands in proportional relationship to the light intensity of the laser beam irradiated on the recording medium 26. Hence, when a predetermined recording optical power or a predetermined reproduction optical power has been set with respect to the recording medium 26, the detection output released by the photodiode 29 varies with every optical head 21 within the above fluctuation range.
When the optical head 21 is adopted for recording information on the recording medium 26, the intensity of the light coming out from the objective lens 25 needs to be 5 to 10 times that of reproduction. Therefore, it is a prerequisite that no saturation occurs in any of the circuits following the photodiode 29 in order to correctly control the intensity of the light going out from the objective lens 25 during recording. The photodiode 29 has a wide dynamic range and usually does not get saturated, but circuits connected to the photodiode 29 such as the APC circuit, etc. are not able to release outputs above the power source voltage. This point needs to be taken in consideration when designing the circuitry, i.e. the gain of the APC circuit needs to be set such that the emission of a laser beam having a high power during recording does not cause the saturation of the circuit.
Consequently, when as described earlier, the detection output released by the photodiode 29 varies greatly, the gains of circuits such as the APC circuit, etc. need to be set such that no saturation occurs, for the optical head 21 where the detection output released by the photodiode 29 is maximal. On the other hand, efficient use cannot be made of the dynamic range of the above circuits in the optical head 21 where the detection output released by the photodiode 29 is minimal. Namely, if in the optical head 21 the detection output is minimal, the detection output released by the photodiode 29 during reproduction is of a low level and has a low S/N whereby the optical power of the semiconductor laser 22 cannot be adjusted as desired. Moreover, when the detection output released by the photodiode 29 is converted from an analog signal into a digital signal and the optical power of the semiconductor laser 22 is digitally controlled, the accuracy in setting the optical power of the semiconductor laser 22 drops significantly in the optical head 21 where the detection output of the photodiode 29 is small, due to a low resolution per bit.
In addition, lately, efforts for improving the transmission rate of data have given rise to the need for enhancing the transmittance of the beam splitter 24 and thereby increasing the intensity of the light illuminated from the objective lens 25 onto the recording medium 26. However, since it is difficult to contain the accuracy of the transmittance and the reflectance of the beam splitter 24 below .+-.5% as mentioned earlier, this would further increase the fluctuation of the detection output released by the photodiode 29. The conventional optical head 21 thus suffers from the drawback that practically it is difficult to improve the transmission rate of data.
Here, in order to solve problems such as described above, one might think of adjusting the gain of the APC circuit for every optical head 21 manufactured. This requires a procedure for adjusting the gain to be added to the manufacturing process and causes the APC circuit to lose its flexibility. Furthermore, when the optical head 21 is replaced during the maintenance performed after delivery, the above arrangement suffers from the drawback that the gain of the APC circuit needs to be adjusted for the newly installed optical head 21.